The spine is a complex columnar structure that is comprised of vertebral bone and connective tissues. The vertebrae, intervertebral discs, and ligaments are intricately arranged such that the interaction between these structures provides strength and support for the distribution of body forces and flexibility for body motion, as well as protection for the spinal cord. In a diseased or injured spine, this intricate arrangement is disrupted. In many cases, such disruptions can be treated by conservative, non-surgical methods, such as medication, exercise, physical therapy, etc. In some cases, however, more radical, surgical methods are required to treat the pain and other symptoms caused by nerve element compression and unstable intervertebral joints. Such more radical, surgical methods involve the dissection of soft tissue and often the removal of load-bearing structures, such as vertebral bone and intervertebral discs. This can lead to spinal instability, and it is often necessary to fuse the associated segments (also referred to as levels) in order to restore spinal stability. Internal fixation with instrumentation typically accompanies spinal fusion to provide temporary spinal stabilization and alignment, as well as an environment in which fusion can take place over time.
A variety of internal fixation systems have been developed to provide temporary spinal stabilization and alignment. These internal fixation systems can be defined as anterior or posterior assemblies, depending on how and where they are implanted with respect to the spine. Anterior assemblies, such as total intervertebral disc replacement assemblies and the like, are coupled to the anterior (front) portion of the spine, while posterior assemblies are coupled to the posterior (rear) portion of the spine, using various pedicle screws and rigid rods, for example. These posterior assemblies typically include adjacent pairs of screws that are inserted through the pedicles and into the vertebral bodies at predetermined angles and depths. Pairs of parallel, longitudinally-aligned rigid rods are then disposed through and/or attached to the adjacent pairs of pedicle screws, essentially creating an immobilizing frame or support structure. Disadvantageously, although providing temporary spinal stabilization and alignment, these internal fixation systems often require an open posterior insertion procedure with a typical incision and muscle retraction, destroy significant portions of the pedicles (facets), result in an undesirably limited range of motion (ROM), are difficult to revise and/or extract, and can lead to adjacent-level degenerative disc disease.
One conventional system for the stabilization of the segments of the cervical, thoracic, lumbar, and sacral spine as an adjunct to conventional spinal fusion (incorporating autogenous bone grafts only)—formerly also using fusion cages, various pedicle screws, and/or rigid rods—is the Dynesys® Dynamic Stabilization System (Zimmer Spine, Minneapolis, Minn. USA). This system uses a plurality of titanium alloy pedicle screws (disposed lateral to the facets of adjacent vertebrae, on either side of the facets) through which a pair of parallel, flexible polyethylene terepthalate (PET) cords are threaded, the cords secured to the plurality of pedicle screws subsequent to proper sizing. A pair of flexible polycarbonate urethane (PCU) spacers are disposed about the cords. The push-pull interaction between the cords and the spacers stabilizes the intervertebral segment. In addition, the tension of the cords decreases post-operatively, due to body temperature warming, repetitive deformation, etc., resulting in a controlled increase in ROM. Advantageously, the system allows for an open posterior insertion procedure with a typical incision and muscle retraction or a minimally-invasive insertion procedure, preserves significant portions of the pedicles (facets), and results in an improved ROM (at rest, in flexion, and in extension). Disadvantageoulsy, the system is difficult to revise if the pedicle screws become loose.
Thus, what are still needed in the art are improved pedicle screw-based dynamic posterior stabilization systems and methods that are used to stabilize the segments of the cervical, thoracic, lumbar, and sacral spine as an adjunct to, or in place of, conventional spinal fusion using bone grafts—formerly also using fusion cages, various pedicle screws, and/or rigid rods. Advantageously, the pedicle screw-based dynamic posterior stabilization systems and methods of the present invention have a more physiologic dynamic interface that allows for more “normal” spine motion, as well as a decreased incidence of pedicle screw loosening.